Traditionally, digitally controlled color ink jet printing capability is accomplished by one of two technologies. Both require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the print head. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a receiving medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e., cyan, yellow and magenta, are used because these colors can produce, in general, up to several million perceived color combinations.
The first technology, commonly referred to as “drop-on-demand” ink jet printing, typically provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the print head and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle.
The second technology, commonly referred to as “continuous stream” or “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no print is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as catcher, interceptor, or gutter). When print is desired, the ink droplets are directed to strike a print media.
Typically, continuous ink jet printing devices are faster than drop-on-demand devices and produce higher quality printed images and graphics. However, each color printed requires an individual droplet formation, deflection, and capturing system.
U.S. Pat. No. 1,941,001, issued to Hansell on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al. on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous ink jet.
U.S. Pat. No. 3,416,153, issued to Hertz et al. on Oct. 6, 1963, discloses a method of achieving variable optical density of printed spots in continuous ink jet printing using the electrostatic dispersion of a charged droplet stream to modulate the number of droplets which pass through a small aperture.
Conventional continuous ink jet printers use electrostatic deflection by a charging tunnel and deflection plates. U.S. Pat. No. 4,346,387, issued to Hertz on Aug. 24, 1982, discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a droplet formation point located within the electric field having an electric potential gradient. Droplet formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect droplets.
U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are controlled, resulting in short filaments and longer filaments. A flow of air is across the paths of the fluid at a point intermediate to the ends of the long and short filaments affects the trajectories of the filaments before they break up into droplets. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member.
U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and to deflect those ink droplets. A print head includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a receiving medium, while non-printed ink droplets flow along a non printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher.
The use of an air stream has been proposed to separate ink drops of a plurality of volumes into spatially differing trajectories. Non-imaging droplets, having one grouping of volumes, is not permitted to reach the image receiver, while imaging droplets having a significantly different range of volumes are permitted to make recording marks on the receiver.
It has been found that good discrimination between large volume droplets and small volume droplets is generaly obtained when the volume of large droplets is about three times greater than the volume of small droplets. As the packing density of nozzles on a print head increases (the nozzles get closer together), adjacent large drops may actually touch one another during flight. If they touch, the droplets will coalesce. Clearly, this would have a negative effect on the printed image were the large droplets selected to reach the receiver, but coalescence could be a problem in the guttering process if the large droplets were selected to be non-printing.
It is desirable to integrate a high density of closely spaced nozzles on a print head. Thus, there is a opportunity to provide a modified ink jet print head and printer having simple control of individual ink droplets with an increased amount of physical separation between large droplets.